Gelled liquid hydrocarbon fluids have been utilized in a variety of treatments for subterranean formations penetrated by well bores, including stimulation activities such as fracturing and/or gravel packing. Such hydrocarbon fluids must have a sufficiently high viscosity to generate a fracture of sufficient dimensions and also to carry the proppant particles into the fracture. Hydrocarbon fluids are frequently gelled by use of phosphate containing gelling agents, particularly phosphate acid ester gelling agents. These agents have been popular because of their effectiveness and comparatively low cost.
One aspect of well treatment processes is the “cleanup”, e.g., returning and removing used fluid from the well after the treatment has been completed. Returned fluids are also useful to carry and remove waste materials, excess proppant and the like from the well. Techniques for promoting cleanup often involve reducing the viscosity of the treatment fluid as much as practical so that it will more readily flow toward the wellbore. This is called “breaking” the fluid. Breaking agents, or “breakers” are specific to the type of treatment fluid being used. Gel breakers are commonly used for conventional polymer based fluids used in stimulation and other activities since leaving such a high viscosity fluid in the formation would result in a reduction of the formation permeability and, consequently, a decrease in the well production. The most widely used breakers are oxidizers and enzymes. The breakers can be dissolved or suspended in the liquid (aqueous, non-aqueous or emulsion) phase of the treating fluid and exposed to the polymer throughout the treatment (added “internally”), or exposed to the fluid at some time after the treatment (added “externally”). Breaking can occur in the wellbore, gravel pack, filter cake, the rock matrix, in a fracture, or in another added or created environment. See, for example, U.S. Pat. No. 4,741,401 (Walles et al.), assigned to Schlumberger Dowell and incorporated herein by reference, for a detailed discussion of breaking activities.
Not all undesirable products are removed with the clean up. Some can remain in the well and be dissolved in or carried by the oil produced by the well. In recent years, problems in downstream processing of crude oil have been encountered, such as plugging of the refinery towers which process the hydrocarbons. These plugging problems can cause build up in the refinery towers and their trays, and removal of such plugging requires shut-down of the affected towers. Upon investigation, the plugging materials have frequently been found to be high phosphorous compounds. Such compounds have been identified as being associated with the aforementioned phosphoric acid ester gelling agents. The phosphoric acid gelling agents are believed to contain volatile phosphorous compounds and/or their precursors which release volatile phosphorous compounds into the wellbore fluids and produced oil. The volatile phosphorous material present in the crude oil products delivered to refineries. The phosphorous condenses onto distillation tower trays during refining. This condensation builds up and results in the plugging of those trays. Excess plugging can lead to a shut down of the towers. Further, volatile phosphorous compounds can carry over and contaminate the products produced by the towers.
Because shut-down of plugged refinery towers causes expense in lost time and cleaning procedures, and because contamination of refined products is highly undesirable, it would be advantageous to remove more of the phosphorous compounds with the return treatment fluid, leaving less phosphorous present in the crude oil sent to the refinery.
It has now been found that the addition of certain delayed breaker compounds to a gelled oil crosslinking with phosphorous will break the fluid and that, surprisingly, using certain amounts of such breaking compound will also cause the phosphorous to change from a volatile to a non-volatile form, reducing the amount of free phosphorous carried by the reflow liquid and the well products which can then be responsible for plugging refinery towers. Breakers may be delayed by various means including the use of surface treatments and the like.